1. Field of the Invention
The present invention is directed to contrast agents, in particular, polyelectrolyte coated metal oxide contrast agent particles for use in MR, X-ray, EIT and magnetometric studies, especially where such metal oxide particles exhibit superparamagnetic behaviour.
2. Background of the Information
The use of contrast agents in medical diagnostic techniques to enhance tissue contrast or to facilitate the study of body processes is well established. The manner in which contrast enhancement occurs varies from imaging modality to imaging modality but in magnetic resonance imaging most of the conventional contrast agents derive their contrast enhancing power from their effects on the tissue selection times.
One of the great advantages of MR imaging is the high degree of intrinsic tissue contrast present from tissue relaxations times. The initial belief was that even without added contrast agents the relaxation parameters could be used for discrimination between normal and malignant tissue (see Damadian, Science 171:1151-1153 (1971)). However, it has become clear since the first MR images were produced by Lauterbur (see Nature 242:190-191 (1973)) that a consistent differentiation of abnormal tissue from normal was impossible. Therefore, there has for some time now been great interest in the use of materials which improve contrast by affecting the key contrast parameters. Lauterbur's group was first to describe the use of MR contrast agents in animals (see Lauterbur et al., Frontiers of Biological Energetics, New York, Academic Press 1978; page 752). The clinical diagnostic potential of an intravenously administered contrast agent was demonstrated by Carr et al. in 1984 (see Carr et al., AJR 143:215-224 (1984)) and the first MR contrast agent, GdDTPA, received approval for clinical use in 1988.
Today it is well documented that GdDTPA and similar substances, like GdDTPA-BMA, GdHPDO3A and GdDOTA, are safe and beneficial for enhanced MR imaging of the central nervous system. Due to their low molecular weight and hydrophilic properties, these metal chelates distribute extracellularly and are rapidly cleared by the kidney. Currently, other contrast agents are being developed with improved pharmacokinetic properties, allowing a more specific organ or disease distribution.
There are, generally speaking, two approaches that may be used to improve the delivery of a MR contrast agent to the target region. In the conventional one, efforts are directed towards using paramagnetically labelled naturally occurring or synthetic molecules or macromolecules with a specific accumulation or localization (e.g. hepatobiliary agents, blood pool agents, porphyrins). An alternative is to use stronger magnetic labels, such as superparamagnetic particles, which accumulate in the desired site by virtue of their particulate nature or by use of their binding to target specific molecules. In general, the diagnostically profitable target/background ratios of the superparamagnetic agents are significantly higher than those of the paramagnetic agents and the superparamagnetic agents can therefore be detected at very low tissue concentrations (see Weissleder et al., Magn. Reson. Quart 8:55-63 (1992)).
The application as MR contrast agents of superparamagnetic agents derives from their ideal combination of a large effect on tissue signal intensity which results in powerful contrast enhancement, and their highly specific targetability. The potential targets of particulate agents are many, depending on the administration route and the physicochemical properties of the particulate material, in particular the particle size and surface characteristics. Their two main applications are by enteral administration for gastrointestinal investigations, and by parenteral administration for investigations of the blood pool compartment and/or the reticuloendothelial system and regions of its anatomical distribution, e.g. the liver, spleen, bone marrow, and lymph nodes. Ultrasmall iron oxide particles with a diameter of less than approximately 30 nm have a relatively long intravascular half life when compared to larger conventional iron oxide particles. In addition to the T2 shortening typically associated with the iron oxide particles, ultrasmall particles also produce T1 shortening thereby increasing signal within the vessels. Recent advances in particulate agents have also made targeting with receptor ligands or antibodies/antibody fragments possible. A brief summary of described applications of different superparamagnetic agents is given in Fahlvik et al., JMRI 3:187-194 (1993).
Up to the present, much of the work carried out in superparamagnetic contrast agents has focused on optimizing their contrast efficiency and biokinetics. Little attention has been paid to pharmaceutical formulation related issues or to safety aspects of the particle preparations.
However for parenteral particulate preparations, adequate contrast efficacy and biokinetics are not in themselves sufficient and certain problems have been encountered. Thus, for example, the conventional iron oxide-dextran preparation, which has been tested in clinical trials, has been shown to have a low colloidal stability. The particles must be redispersed and/or diluted and filtered immediately before use and the preparation is administered as a slow infusion through an in-line filter to avoid severe toxic effects.
While particulates can be coated to give adequate stability, the particle surface area of a parenteral particulate agent is substantial and we have found that coating agents conventionally thought to be wholly innocuous, such as for example the storage-type polysaccharides starch and dextran and their derivatives can themselves have a deleterious effect on cardiovascular parameters, platelet depletion, blood coagulation time and on the complement system.